

Feb 12, 2012, 05:37 PM  

Quote:
To appear that it is traveling at the same speed as its fullscale counterpart, a model must traverse the observers fieldofview in the same amount of time as the fullscale  when the model is at the distance where it's apparent size is the same as the fullscale. It is a fact that scale speed = the square of the scale factor. The formula is used in movies all the time when they're using scale models. It is also the rule that's used for judging scale speed at RC competitions. I suggest that you read this article: Understanding Scale Speed  by Bob Boucher. Joel 

Feb 13, 2012, 12:46 AM  
United States, CA, Ridgecrest
Joined Dec 2011
39 Posts

TurboParker: I understand your position and our difference is only semantic. Boucher's analysis is correct, but he's talking about something a bit different than I. He's including scaling time, which movies do by shooting with high speed cameras to slow the action and make scale models respond with apparent full scale physics. Notice he mentions scale time. When we're at the field watching our own planes in real time, we can't change time to slow our planes down. To get the plane moving at the same apparent rate as a real one it has to transit it's own length in the same delta time, in real time, as the real one, so scale speed is directly proportional to scale factor.
However, this is nearly impossible because the aerophysics cause small models to have to move much faster to stay airborne. The weight goes as scale cubed, the wing area as scale squared, so the wing loading goes as scale. The stall speed is proportional to the square root of the wing loading so it decreases as the square root of scale factor, but that's in real mph. The scale speed is real speed divided by scale factor, so it goes UP as the inverse of the square root of the scale factor and model planes have to takeoff and land much faster than scale. Maybe the square root factor is included in competitions to acknowledge that limitation, i.e. performance and loops etc. are judged in "scale time" which is faster than real time by the square root of the scale factor. The discrepancy is purely semantic: Boucher is scaling time; some of us want scale speed in real time (but we're not likely to get it). At least the PZ micro Spit is helped by its very low wing loading (est. 16 gm/dm^2) for an estimated stall speed of about 10 real mph. 
Feb 13, 2012, 04:37 AM  


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Feb 14, 2012, 02:36 PM  

Quote:
Joel 

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